It is well known to provide hydrostatic ground drives for work machines such as agricultural combines and the like. Such drives typically include one or more fluid pumps operable for supplying a flow of pressurized fluid to a primary drive including at least one fluid motor for operating a set of primary movers, typically the front movers, of the machine. Additionally, it is known to provide such drives in combination with multiple speed mechanical transmissions in driven relation to the at least one fluid motor and connected in driving relation directly with the primary movers. These clutchless transmission systems require the work machine to come to a complete stop prior to shifting between speed ranges. The multiple speed ranges or gears of the mechanical transmission include at least one low speed range suitable for performing work functions in a field and at least one high speed range suitable for travel over public roads. In addition, when the transmission is engaged in a low speed range and the machine is experiencing high propulsion torque requirements, a power assist mode may be engaged to divert a portion of the flow of pressurized fluid to a secondary drive including one or more fluid motors in driving relation to a set of secondary movers, typically the rear movers, of the machine. In other words, additional propulsion torque is provided by the secondary movers when a portion of the flow of pressurized fluid is divided between the primary fluid drive and the secondary fluid drive. For example, the power assist mode is typically employed to provide additional torque from the rear axle drive of a combine in muddy conditions during field operations and for improved steering on side hills.
Problems and shortcomings of work machines having clutchless mechanical transmission systems in combination with presently known hydrostatic ground drives include the inability to provide additional propulsion torque when the transmission is engaged in a high speed range. For example, when the work machine is performing an operation in which it will be driven at high speed, the transmission is necessarily engaged in a high speed range. However, if the machine encounters muddy or hilly conditions during the operation, the propulsion torque required by these conditions may exceed the propulsion torque available from the primary drive alone, resulting in extremely poor operating efficiency, excessive hydraulic fluid heat, hydraulic component wear, slow acceleration, or even stalling. In this situation, the machine must be stopped, and the mechanical transmission shifted into a low speed range so the power assist mode can engage to provide additional propulsion torque from the secondary drive. Once out of the mud or at the crest of the hill, the machine must again be stopped and the mechanical transmission shifted into a high speed range to continue the high speed operation.
Another example in which these problems and shortcomings have been observed is in connection with transporting a work machine over the road. When climbing out of a field, the work machine experiences a high propulsion torque requirement, a situation in which additional propulsion torque from the secondary drive may be required, so the transmission must be engaged in a low speed range. When traveling over the road, however, the transmission is preferably engaged in a high speed range in order for the work machine to reach road speeds. Using presently known systems, the work machine must pull out of the field onto the road with the transmission in a low speed range so additional propulsion torque is available from then secondary drive to propel the machine up the hill. Once on the road, the machine must come to a complete stop in order to shift the transmission into a high speed range for traveling at road speeds.
It would be advantageous for the work machine to have additional propulsion torque to be able to pull out of the field onto the road and accelerate to road speeds without the need to stop on the road to shift the transmission from a low gear to a high gear. Using presently known systems, however, the choices include driving at road speeds with the transmission in a low speed range which causes excessive hydraulic system wear, or climbing out of the field with the transmission in a high speed range and risk experiencing propulsion torque requirements that can not be met by the primary drive.
What is sought therefore is a work machine having a clutchless mechanical transmission in combination with a hydrostatic ground drive which automatically provides increased propulsion torque availability when the transmission is engaged in a high speed range, and which overcomes one or more of the problems and shortcomings set forth above.